Compressed cellulosic product and method of making same



May 2, 1 944. H. K. LINZELL 2,343,081

COMPRESSED CELLULOSIC PRODUCTS AND METHOD OF MAKING SAME Filed- Jan. 24, 1941 COMPRESS CELLULOSIC HEAT AND COMPRESS MATERIAL-EXCLUDE OXGEN (.ELLULOSIC MATERML IN WHILE MATERIAL l5 HOT AB5ENCE OF OXYGEN 0001. WITHOUT 0mm T/an/ HEAT CELLl/LUS/C MATERIAL /IV ABSENCE OF OXYGEN COMPRESS l/EHTED CfMUlOS/ [VI-l TAP/4L IN HBSE/VCE 0F OXYGEN COOLCELLULOS/C METER/0L WflIlE EXCLUDING oxmm RAPMLY cob; amazes/c MflTER/HL 1v NON-OXIDIZ/IVG 50857 AN 7' /(1L OX I08 770 HEAT AND COMPRESS 45440405: MATERIAL COVER 508F465 OFMMEAMA WITH oxva-W- Exa uo/lvti suasmrvcs mvo c004 Patented May 2, 1944 COMPRESSED CELLULOSIC PRODUCT AND METHOD OF MAKING SAME Harry K. Linzell, Long Lake, 111., assignor to United States Gypsum Company, Chicago, 111., a corporation of Illinois Application January 24, 1941, Serial No. 375,769

22 Claims.

The present invention relates to certain improvements in the manufacture of compressed cellulosic products, particularly those which are manufactured under temperature conditions conducive to the formation of autogenously developed.

resinous binders resulting from the partial pyrolysis of the cellulose and/or lignocellulose therein contained.

One of the objects of the present invention is to produce compressed cellulosic and/or lignocellulosic products at temperatures which ordinarily are within the range at which oxygen would deleteriously afiect such materials, as by oxidizing and so darkening the same, but preventing such oxidation by the expedient of excluding from them any oxidizing agency during the time they are at the elevated temperature.

A further object of the invention is to manufacture a hardboard having two smooth surfaces which have been kept relatively light colored by the expedient of avoiding undue oxidation of such surfaces either by excluding oxygen therefrom during the heating steps or Whil the materials are hot or by rapidly cooling the materials after access of oxygen or oxidizing gases thereto is permitted, so as to rapidly lower the temperature of the materials to the nonoxidizing point, thereby preventing their undesired oxidation.

A further object is to permit preheating of the material in an inert atmosphere, thereby bringing the same up to or close to the point of softening of the ligneous constituents, thereby permitting a shorter pressing, time. This also will avoid the possibility of ignition of the material.

The manufacture of hot pressed cellulosic and lignocellulosic products is an already well established industry, and the manufacture of hard boards such as synthetic wood or other consolidated lignocellulosic materials has already been fully described in the prior art. Broadly speaking, such manufacture includes the consolidation of cellulosic or lignocellulosic material at tem- Deratures which lie somewhere between 350 F. and one short of the development of free carbon, the upper temperature in general being somewhere around 600 F., dependent upon the time of pressing. Within such a temperature range, lignocellulosic material undergoes certain pyrolytic changes by which resinous substances, of a nature not fully understood, are autogenously produced from the material, acting as binders to keep the fibers thereof together. It has already been prposed to take a light porous blank of lignocellulosic material, which may be exemplified by fibrated wood, and to compact such a blank by means of highly heated platens to form what is known commercially as hardboard, semihardboard, and the like. It has been customary to use a temperature of about 425 to 485 F. for this purpose. Under such conditions there is a considerable darkening of the board, which is particularly noticeable on the surfaces thereof. This darkening, while due in part to the pyrolysis of the lignocellulosic material, is considerably enhanced by the further oxidationathereof during the time in which it is allowed to cool if in contact with atmospheric oxygen, particularly when pressed at 450 F. or higher. While this dark color is quite Pleasing for some purposes, the lighter surfaced product possesses several advantages desired by the consumer.

Therefore, in accordance with the present invention the undesired darkening of such boards is avoided, or at least considerably minimized, by working under conditions, or employing means, that will hinder the development of the aforesaid color. This can be accomplished by quite a number of means within the scope and purpose of the present invention.

Figs. 1 to 6 are in the nature of flow sheets illustrating procedures according to the present invention.

Figs. 7 and 8 are cross sections of compacted cellulosic boards according to my invention, having, respectively, two unoxidized surfaces, and one unoxidized surface and one oxidized surface.

As illustrated in Fig. 1, it will be self-evident that if during the heating stages, or at least during those times when the lignocellulosic material is at a temperature conducive to oxidation, oxygen or atmospheric air is kept out of contact there with, the oxidation will be substantially prevented.

According to one exemplification of the present invention, see Fig. 2, a comparatively light porous blank, or even loose llgnocellulosic material, may be heated without pressure in a, nonoxidizing atmosphere to a temperature productive of the desired autogenously developed binders; that is, a temperature between, let us say, 350 to 600 F. The length of time that the material is subjected to this temperature is roughly inversely propor tional to the temperature. By this it is intended to convey the thought that a higher temperature requires a shorter time and a lower temperature requires a longer time. Thus, at 350 F. a period of one-half to one hour may be required to produce the desired amount of pyrolytic change in the lignocellulose, at 425 F. 10 to 15 minutes may be sufficient, while at 450 F. 5 to 10 minutes may suffice. For example, lignocellulosic material l which may or may not first have been made, by means now known, into a more or less coherent blank, may be heated in an inert atmosphere to a temperature of, let us say, 425 F. This inert atmosphere may consist of nitrogen, combustion gases. helium. argon, or even steam, the whole object being to subject the lignocellulose to the desired thermolytic decomposition without, however,

permitting its oxidation or burning. The thus preheated blank may then be rapidly transferred between platens of a hydraulic press, in which it is compacted at a pressure of from 500 to 2500 pounds per square inch, the platens preferably,

material will take place, and self-evidently the surface contact between the blank and the platens will prevent access of oxygen to the surfaces during the compacting or pressing step. Upon release of the pressure, however, precautions must be taken to avoid access of oxygen. This can be accomplished by, for, instance, blowing a blast of nonoxidizing gas across or upon both surfaces of the pressed board immediately upon the release of the pressure and during the renioval of the board from the press. Also as shown in Fig. 3 the surfaces of the board may haveapplied thereto a covering of, for example, metal foil, or other comparatively heat resistant material, provided only that the substance employed will prevent access of oxygen to the surfaces of the board.

As an equivalent means, see Fig. 4, the boards upon withdrawal from the prefi may be rapidly pushed into a second press w ch is kept cool,

' for example by the expedient of circulating a cooling medium through the platens thereof, be-

' tween which the boards are lightly pressed and held until they have reached a temperature below which any further oxidation upon contact with oxygen will be substantially precluded. In place of such a cooling press. cooled rollers might be employed for a similar purpose, the yvhole idea being so rapidly to cool the boards that oxidation to any substantial extent need not be feared.

As a further alternative, see Fig. 5, the surfaces of the blanks, prior to the compacting step, might be lined with, for instance, metal foil or thin sheet metal during the preheating process, the pressing then being accomplished by means of heated platens, whereafter the boards may be allowed to -cool naturally, following which the metal foil can be stripped from the blanks, leaving substantially nonoxidized surfaces. This method also lends itself to the deliberate ornamentation or surface marking of the boards. For example, metal foil might be applied to the blanks in predetermined areas, such as patterns, reading matter, or the like, whereupon the boards might be pressed and then taken from the press while still hot but with predetermined areas protected from oxidation by the presence of the covering. Upon subsequent removal of the covering,'the

. markings will appear in a lighter color against,

' the darker background.

As a further example, the boards may be preheated either in an inert atmosphere or between suitable protective sheets until they have acquired a temperature of, say, 400 E, which might take minutes, whereafter they may be pressed for a relatively short time, for instance 30 seconds at 475 F., seconds at 500 l"., or almost instantaneouslyat 600' F. Such preheating may be in suitably heated inert gases or vapors. These may be any gas devoid of available oxygen, thus including steam, carbon dioxide, combustion gases, nitrogen, hydrogen. etc. 'If the board or mat is not first preheated, such high pressing temperatures are likely to effect merely theproper bonding of the outer layers in a short time, leaving however the interior of the product in an insumciently bonded condition. While there may be some circumstances in which this might be advantageous, such a product, when subjected to moist conditions, may exhibit a tendency to delaminate the surface from the core.

\ A further alternative is to preheat the-porone board blank under conditions avoiding oxidation to a suillciently high temperature for a length of time consistent with the temperature used to activate or soften the same, whereafter an almost instantaneous pressure, such as that exerted by a roller, or a quick-acting toggle or hydraulic press, can be used to compress the material. This will permit the carrying out of a continuous process.

A still further alternative, see Fig. 8, is to place the blanks in superficial contact with the Plusing platens themselves until, as a result of the heat radiated thereto, the blanks acquire the desired pressing temperature.' A slight amount of oxidation will be effected by virtue of the atmospheric oxygen contained in the interstices of the blanks, but such oxygen is soon used up,.

and therefore the-oxidation is definitely limited. Upon then applying the full pressure of the press. consolidation will be effected. It therefore remains merely to protect the thus pressed boards against any further oxidation by the expedient of blowing a current or blast of nonoxidizing gases, or even cool air, against the press and across the surfaces of the boards as soon as the press is opened; or as a further alternative the entire press might be placed in a housing in which a nonoxldizing atmosphere is maintained during the entire pressing operation. The last mentioned expedient is comparatively simple; thus a hydraulic press of well-known type of construction may be housed in a heat insulatin housing in which an atmosphere of combustion gases substantially devoid of oxygen is maintained, orthe atmosphere within the press housing may consist of nitrogen, carbon dioxide, steam or inert gas. It will. of course, be advisable to employ a noncombustible gas; and although hydrogen will work, it would berather dangerous. Such a press might be charged through gates or flaps, which might be hinged or made out of flexible material such as fabric, through which go the boards could be pushed into the press without admitting any substantial amounts of atmos-- pheric oxygen; .and the boards could be automatically withdrawn from the press through similar flaps or gates after completion of the operation, being then dischargedinto a second, adjacent chamber also filled with a nonoxidizing atmosphere in which the boards would be allowed to cool. Artificial means for cooling the boards while in this nonoxidlzing cooling chamber might be resorted to if desired.

If it is desired to avoid even the oxidation which might be the result of the presence of air within the interstices of the blanks, they might preliminarily be placed in a chamber from which 75 the air is evacuated, the vacuum then being broken by means of a suitable inert gas, for example carbon dioxide, nitrogen, or combustion gases, so that even the interstices of the boards will be filled with a nonoxidizing gas. If the boards thus impregnated as it were with an inert gas are rapidly placed between the platens of the ordinary hydraulic press at present employed for the making of hardboard, practically no oxidation will take place, because the rate of diffusion is so slow that during the few minutes required for the transfer of the boards to the press the inert gases would not be removed from their interstices. The platens of the press would be depended upon to exclude atmospheric oxygen during the pressing step. Under such conditions the blowing of an inert gas across the surfaces of the boards upon removal from the press until they have been cooled below oxidizing temperatures is generally sufiicient to prevent more than a slight superficial oxidation. Of course, these various steps may be combined with the expedient of transferring the pressed boards rapidly into a nonoxidizing atmosphere in which they are allowed to cool.

Broadly speaking, therefore, the present invention constitutes a distinct improvement in the manufacture of hardboard by avoiding the ordinary oxidizing efiect which gives it a dark color; and any means which will exclude oxygen, whether they be of a solid material or of a fiuid material, are to be considered as equivalents for the purposes of the present invention.

Other modifications which will occur to those who are familiar with the art of makin hardboard are to be construed as within the scope of the present invention, such for example as the carrying out of the entire process in a vacuum instead of an inert atmosphere, the gases given off during pressing and heating being removed. In this case, care must be used when the press is released to prevent the formation of blisters within the board. It sometimes is found desirable to break the vacuum at this point to minimize the formation of blisters.

While the invention is primarily directed to the manufacture of lignocellulosic products, it is obvious that in this case it will apply to a considerable extent to the manufacture of cellulosic materials, in which case a small amountof binder may have been added for the purpose of bonding fibers together.

As an example of the carrying out of the present invention, but without intending to limit it thereby, the actual manufacture of a light colored hardboard will now be described. Wood such as cottonwood may be pulped in any suitable manner, as for instance by grinding it wet by grindstones. The resulting pulp, after removal of the insufliciently ground larger pieces, is sized, for instance with rosin size and alum, and is then formed, at the proper state of dilution, into an insulating board by a suitable machine. The wet lap thus produced is dried at, say, 250 to 275 F. At this stage, protection against oxidation is not necessary, as the temperature is substantially below the point where any amount of undesirable oxidation would take place. The blanks thus produced, after being cut into the desired sizes, are then heated to a temperature of, for example, 375 to 450 F., or higher, in a nonoxidizing atmosphere. most advantageously produced by burning gas or oil under such conditions as to use up substantially all of theoxygen contained in the air used for supplying. the oxygen to the burners.

This is' A certain amount of recirculation of the combustion gases can be resorted to in order to insure the substantial absence of oxygen in the interior oi the heating chamber. Alternatively, superheated steam, nitrogen, carbon dioxide, or the like, may be heated by heat exchange and depended upon to bringthe blanks or insulating boards to the aforementioned temperature. This temperature can'be maintained for a period of from 5 to 10 minutes, allowing ample time for the insulating boards-to acquire this temperature, whereby a slight pyrolysis of the therein contained lignocellulosic fibers will be effected, with the concomitant formation of decomposition products which act as eventual binders for the boards. The heated boards are then as rapidly as possible transferred, either automatically or otherwise, between the platens of a hydraulic press, the platens being heated to a temperature of 350 to 600 F. The press is then 0per ated to compress these heated insulating boards to form the hardboard, which is usually about A; inch thick. However, by using suitable stops or by proper control of the pressure, the boards may be made into semihardboard, say, inch thick. In any event, upon opening the press, a blast of cold nonoxidizing gas is directed over both surfaces of the boards, this blast being continued until the boards have cooled to below approximately 350 F., for at that temperature they are substantially resistant to further oxidation. Boards thus manufactured have a very pleasing light color on both sides, see Fig. 7, while boards which have not been made with the present precautions present a dark brown. almost roasted coilee colored, appearance. As already indicated, the press may be suitably housed so as to permit the maintenance of an inert atmosphere around it.

Another way of proceeding is to heat the porous insulating blanks to a temperature of from, say, 375 to 450 F. in an inert atmosphere, whereafter the heated blanks are pressed at 475 F. for one minute. By raising the temperature of pressing to 500 F., the time-need only be about 15 to 30 seconds. At 600 F. a mere almost instantaneous application of sufiicient pressure will accomplish the desired ,consolidation of the blanks into a hard product.

It will also be understood that the prevention of the oxidation might be limited to one side of the boards, thus producing boards having a light color on one side and a dark color on the other, see Fig. 8, so that the user will have a choice of which side to expose to view.

Another method of accomplishing substantially the same result is to dry the insulating boards to a moisture content of about from 1% to 3% at a temperature of 275 to 300 F. and then to apply to such relatively dry boards a coating of metal foil or other nonoxidizable sheet material. The boards may then be pressed at the aforementioned temperature of between 350 and 600 F. andthen removed from the press and allowed to cool naturally. This may cause the metal foil or other material to adhere to the boards, in which case they may be used with such a coating. However, the bond between lignocellulosic material and metal foil is comparatively slight, and the latter may easily be stripped from the surfaces, if desired, though it is within the scope of this invention to use a bonding material to aid the bond between the two dissimilar products.

As a still further modification, the materialfor example, the above mentioned mats or even loose dry lignocellulosic material- -may be beated in a 'nonoxidizing atmosphere to a temperature and for a time sufficient to develop the binding properties thereof, as by activation of the ligneous constituents, for example, whereafter the thus treated material may then be compressed at a relatively low temperature, say below 400- F., to effect its consolidation into a hard and relatively dense product. This compression may be exerted by any suitable instrumentality, such as rollers or presses, either mechanically or hydraulically operated. The main consideration is the avoidance of combustion bY the exclusion of combustion-supporting gases, such as oxygen. This may obviously be accomplished either by exhausting the air from the environment in which the material is being treated or by surrounding the material witha substantially inert atmosphere.

What is claimed as new is:

1. In the manufacture of hardboard, the step which comprises counteracting the development of a darkened color on the surface of the board by excluding oxygen therefrom while it is hot.

2. In the manufacture of a relatively dense heat-consolidated cellulosic product the step which comprises counteracting the development of a darkened color thereon by excluding oxygen from it while it is hot.

3. The process of producing a relatively dense cellulosic product which comprises the steps of subjecting cellulosic material to compacting pressure at a temperature between 350 F. and 600 F. whileexcluding oxidizing gases from contact therewith and thereafter cooling the resulting product under conditions substantially avoiding its oxidation.

4. The process of producing a relatively dense cellulosic product which comprises the steps of heating cellulosic material in a nonoxidizing atmosphere to a temperature between 350 F. and 600 F. and then applying suillcient pressure thereto to consolidate the same, said consolidation being effected without permitting contact of oxygen therewith. 1

5. The process of making a consolidated cellulosic product such as hardboard and the like without the development of a dark color thereof which comprises consolidating cellulosic material under pressure at a temperature between 350 F. and 600 F. while avoiding oxidation of the cellulose by exclusion of oxygen while the material is hot until after the consolidation has been accomplished and thereafter so rapidly cooling it as substantially to avoid the oxidation thereof by atmospheric oxygen.

6. Method of avoiding after-darkening of hot pressed cellulosic materials such as hardboard and the like which comprises avoiding the development of oxidation products thereon by exclusion of oxygen during pressing and while the product is hot.

'7. Method of avoiding after-darkening of hotpressed cellulosic materials which comprises so rapidlycoolingthem after the hot-pressing step as to avoid the oxidation thereof by atmospheric oxygen.

8. Process for manufacturing hardboard from lignocellulosic material which comprises the steps of preheating a relatively porous blank of such material in an inert atmosphere, at a temperature of from 350 F. to 600 F., thus avoiding any substantial oxidation thereof, theremeans of thereto applied consolidating pressure under conditions that avoid any substantial oxidation, and thereafter cooling the resulting product under conditions avoiding its oxidation.

9. Process for manufacturing'hardboard from lignocellulosic material which. comprises the steps of preheating a relatively porous blank of such material in an inert atmosphere heated to a temperature of from 350 F. to 600 F., thereupon consolidating the thus heated blank b? means of thereto applied consolidating pressure under conditions that avoid any substantial oxidation, and thereafter cooling the resulting product under conditions avoiding its oxidation as by exclusion of oxygen during the cooling.

10. Process for manufacturing hardboard from lignocellulosic material which comprises the steps of preheating a relatively porous blank of such material 'at a temperature of from 350 F. to 600 F. in an inert atmosphere so as to avoid any substantial oxidation thereof, then consolidating the thus heated blank by means of thereto applied consolidating pressure under conditions that avoid any substantial oxidation and thereafter cooling the resulting product under conditions avoiding its oxidation as by so rapidly cooling the same as to cause it to attain a non-oxidizing temperature before substantial oxidation has taken place.

11. Process for manufacturing hardboard from lignocellulosic material which comprises the steps of preheating a relatively porous blank of such material in an inert atmosphere at a temperature of from 350 F. to 600 F., thus avoiding any substantial oxidation thereof, then consolidating the thus heated blank by means of thereto applied consolidating pressure under conditions that avoid any substantial oxidation, and thereafter cooling the resulting product under conditions avoiding its oxidation as by keeping the surfaces thereof covered with an oxygen excluding substance.

12. Process for manufacturing hardboard from lignocellulosic material which comprises the steps of preheating a relatively porous blank of such material under nonoxidizing conditions at a temperature of from 350 F. to 600 F. while avoiding any substantial oxidation thereof, then consolidating the thus heated blank by means of thereto applied consolidating pressure under conditions that avoid any substantial oxidation, and thereafter cooling the resulting product under conditions avoiding its oxidation as by keeping the surfaces thereof swept free of oxidizing gases by a blast of inert gas.

13. Process of manufacturing light-colored hardboard from lignocellulosic material which comprises compacting such material under pressure at a temperature between 350 F. and 600 F. and thereafter cooling the resulting product, the compacting and cooling being effected under conditions avoiding its substantial oxidation.

' compacting and cooling being effected under conupon consolidating the thus heated blank by ditions avoiding its substantial oxidation by excluding oxygen therefrom during the cooling.

15. Process of manufacturing light-colored hardboard from lignocellulosic material which comprises compacting such material under pressure at a temperature between 350 F. and 600 F. under conditions that avoid any substantial oxidation and thereafter cooling the resulting product under conditions avoiding its substantial oxidation by so rapidly cooling the same as to cause it to attain a nonoxidizing temperature before substantial oxidation has taken place.

16. Process of manufacturing light-colored hardboard from lignocellulosic material which comprises compacting such material under pressure at a temperature between 350 F. and 600 F. and thereafter cooling the resulting product, the compacting and cooling being eifected under conditions avoiding its substantial oxidation by keeping the surfaces thereof covered with an oxygen-excluding substance.

1'7. Process of manufacturing light-colored hardboard from lignocellulosic material which comprises compacting such material under pressure at a temperature between 350 F. and 600 F. and thereafter cooling the resulting product, the

compacting and cooling being effected under conditions avoiding its substantial oxidation by keeping the surfaces thereof free of oxidizing gases by a blast of inert gas.

18. Hardboard consisting essentially of consolidated lignocellulosic fibers bonded by therefrom autogenously developed resinous binders and having a color in the interior darker than the natural color of the lignocellulosic fibers from which the board is made, and having at least one surface having a color substantially no darker than the color of the interior.

19. Hardboard as defined in claim 18, in which the major portion of both of the outer surfaces is substantially no darker than the interior of the board.

20. The process of producing a relatively dense cellulosic product which comprises the steps of heating cellulosic material in an atmosphere of steam to a temperature between 350 F. and 600 F., and then applying sufiicient pressure thereto to consolidate the same, said consolidation being effected without permitting contacting of oxygen therewith.

21. Process for manufacturing hardboard from lignocellulosic material which comprises the steps of preheating a relatively porous blank of such material in an atmosphere of steam at a temperature of from 350 F. to 600 F., thus avoiding any substantial oxidation thereof, thereupon consolidating the thus heated blank by means of thereto applied consolidating pressure under conditions that avoid any substantial oxidation,

, and thereafter cooling the resulting product under conditions avoiding its oxidation.

22. Process for manufacturing hardboard from lignocellulosic material which comprises the steps of preheating .a relatively porous blank of such material at a temperature of from 350 F. to 600 F. in an atmosphere of steam so as to avoid any substantial oxidation thereof, then consolidating the thus heated blank by means of thereto applied consolidating pressure under conditions that avoid any substantial oxidation, and thereafter cooling the resulting product under conditions avoiding its oxidation as by so rapidly cooling the same as to cause it to attain a nonoxidizing temperature before substantial oxidation has taken place.

HARRY K. LINZELL. 

